Optical gas sensors usually have one or more radiation sources, which emit suitable radiation over a measured section, wherein the resulting measuring beams pass through the measured section containing the gas to be measured once or several times, and the intensity of the measuring beams weakened by the optical absorption of the gas to be measured is measured by means of one or more radiation detectors. The residual, measured radiation intensity of a gas-specific wavelength or gas-specific wavelengths is an indicator of the concentration of the measured gas.
Such a measuring device is described, e.g., in DE 197 13 928 C1.
The infrared absorption is characteristic for the measurement of the concentrations of many gases, e.g., hydrocarbons (e.g., methane), and it is therefore particularly preferred for the measurement.
A problem arises in the prior-art optical gas sensors from the fact that the optical measured section must be as readily accessible as possible to the gas to be measured in order for a measured signal representative of the current gas concentration at the site of the measurement to be able to be obtained possibly without a time delay.
Since the measured section is usually limited by optical elements, such as windows, mirrors or lenses or is provided with such elements, these elements are also exposed to interfering environmental effects, e.g., rain, snow, dust, aerosols or contaminants in the same manner at the site of the measurement, besides the gases to be measured. These environmental effects lead to disturbances or weakening of the measuring beam and ultimately of the measured signal sent by the radiation detectors.
When the measuring beam is weakened beyond a certain extent, the concentration of the gas to be measured cannot be measured any longer, so that the measuring unit with the radiation detectors will send more or less abruptly an interference signal in this case, especially an electric interference signal in the form of a constant current intensity.